[PATCH v6 21/21] kvm: x86: Disable interception for IA32_XFD on demand

Paolo Bonzini
2022-01-07T18:56:07Z

From: Kevin Tian <kevin.tian@intel.com>

Always intercepting IA32_XFD causes non-negligible overhead when this
register is updated frequently in the guest.

Disable r/w emulation after intercepting the first WRMSR(IA32_XFD)
with a non-zero value.

Disable WRMSR emulation implies that IA32_XFD becomes out-of-sync
with the software states in fpstate and the per-cpu xfd cache. This
leads to two additional changes accordingly:

- Call fpu_sync_guest_vmexit_xfd_state() after vm-exit to bring
software states back in-sync with the MSR, before handle_exit_irqoff()
is called.

- Always trap #NM once write interception is disabled for IA32_XFD.
The #NM exception is rare if the guest doesn't use dynamic
features. Otherwise, there is at most one exception per guest
task given a dynamic feature.

p.s. We have confirmed that SDM is being revised to say that
when setting IA32_XFD[18] the AMX register state is not guaranteed
to be preserved. This clarification avoids adding mess for a creative
guest which sets IA32_XFD[18]=1 before saving active AMX state to
its own storage.

Signed-off-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jing Liu <jing2.liu@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20220105123532.12586-22-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
arch/x86/include/asm/kvm_host.h | 1 +
arch/x86/kvm/vmx/vmx.c | 24 +++++++++++++++++++-----
arch/x86/kvm/vmx/vmx.h | 2 +-
arch/x86/kvm/x86.c | 8 ++++++++
4 files changed, 29 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 6cbf97a2ebc4..89d1fdb39c46 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -647,6 +647,7 @@ struct kvm_vcpu_arch {
u64 smi_count;
bool tpr_access_reporting;
bool xsaves_enabled;
+ bool xfd_no_write_intercept;
u64 ia32_xss;
u64 microcode_version;
u64 arch_capabilities;
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index b8b7f5c7b3df..15e30602782b 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -162,6 +162,7 @@ static u32 vmx_possible_passthrough_msrs[MAX_POSSIBLE_PASSTHROUGH_MSRS] = {
MSR_FS_BASE,
MSR_GS_BASE,
MSR_KERNEL_GS_BASE,
+ MSR_IA32_XFD,
MSR_IA32_XFD_ERR,
#endif
MSR_IA32_SYSENTER_CS,
@@ -764,10 +765,11 @@ void vmx_update_exception_bitmap(struct kvm_vcpu *vcpu)
}

/*
- * Trap #NM if guest xfd contains a non-zero value so guest XFD_ERR
- * can be saved timely.
+ * Disabling xfd interception indicates that dynamic xfeatures
+ * might be used in the guest. Always trap #NM in this case
+ * to save guest xfd_err timely.
*/
- if (vcpu->arch.guest_fpu.fpstate->xfd)
+ if (vcpu->arch.xfd_no_write_intercept)
eb |= (1u << NM_VECTOR);

vmcs_write32(EXCEPTION_BITMAP, eb);
@@ -1978,9 +1980,21 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
break;
case MSR_IA32_XFD:
ret = kvm_set_msr_common(vcpu, msr_info);
- /* Update #NM interception according to guest xfd */
- if (!ret)
+ /*
+ * Always intercepting WRMSR could incur non-negligible
+ * overhead given xfd might be changed frequently in
+ * guest context switch. Disable write interception
+ * upon the first write with a non-zero value (indicating
+ * potential usage on dynamic xfeatures). Also update
+ * exception bitmap to trap #NM for proper virtualization
+ * of guest xfd_err.
+ */
+ if (!ret && data) {
+ vmx_disable_intercept_for_msr(vcpu, MSR_IA32_XFD,
+ MSR_TYPE_RW);
+ vcpu->arch.xfd_no_write_intercept = true;
vmx_update_exception_bitmap(vcpu);
+ }
break;
#endif
case MSR_IA32_SYSENTER_CS:
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index 69dd2f85abdc..f8fc7441baea 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -349,7 +349,7 @@ struct vcpu_vmx {
struct lbr_desc lbr_desc;

/* Save desired MSR intercept (read: pass-through) state */
-#define MAX_POSSIBLE_PASSTHROUGH_MSRS 14
+#define MAX_POSSIBLE_PASSTHROUGH_MSRS 15
struct {
DECLARE_BITMAP(read, MAX_POSSIBLE_PASSTHROUGH_MSRS);
DECLARE_BITMAP(write, MAX_POSSIBLE_PASSTHROUGH_MSRS);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index bde18ca657db..60da2331ec32 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10083,6 +10083,14 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
vcpu->mode = OUTSIDE_GUEST_MODE;
smp_wmb();

+ /*
+ * Sync xfd before calling handle_exit_irqoff() which may
+ * rely on the fact that guest_fpu::xfd is up-to-date (e.g.
+ * in #NM irqoff handler).
+ */
+ if (vcpu->arch.xfd_no_write_intercept)
+ fpu_sync_guest_vmexit_xfd_state();
+
static_call(kvm_x86_handle_exit_irqoff)(vcpu);

if (vcpu->arch.guest_fpu.xfd_err)
--
2.31.1